The invention relates to a controllable quadrature oscillator having a pair of oscillator outputs for supplying a pair of phase quadrature oscillator signals, and a cascade circuit of two quadrature sections, which circuit is incorporated in a regenerative loop, one of the two quadrature sections being arranged between the two oscillator outputs and realizing a 90.degree. phase shift in the regenerative state of the loop.
Such a controllable quadrature oscillator is known, for example, from U.S. Pat. No. 3,936,773.
The cascade circuit of the two quadrature sections is regeneratively fed back from the output to the input for the frequency at which the gain in the loop formed for the quadrature sections and the feedback is one and a phase shift of 90.degree. occurs in each of the two quadrature sections. To this end each of the two quadrature sections of the known quadrature oscillator comprises an integrator and an analog multiplier.
For each frequency within a given frequency range the integrator realizes a 90.degree. phase shift, while the loop gain for a desired frequency within the last-mentioned frequency range is controlled at the value of one by means of the said analog multiplier of each of the two quadrature sections. Since this frequency range determines the oscillator control range and in itself is comparatively narrow (in practice the integrators realize an accurate 90.degree. phase shift only for some frequencies), the known quadrature oscillator comprises a phase correction circuit in which an amplitude controllable part of the output signal of one of the two quadrature sections is added to its input signal. Although this results in a given increase of the oscillator control range, it appears in practice that the control range of the known oscillator is limited at its upper side to a maximum value of the order of 100 kHz and unwanted large deviations in the phase quadrature relation between the two phase quadrature oscillator signals are not prevented by means of the said phase correction circuit. Moreover, comparatively large time constants are required for low oscillation frequencies in the known oscillator. To realize such large time constants in integrated circuits, a comparatively large quantity of chip surface is required so that the known oscillator, particularly for low oscillator frequencies, is less suitable for integration.
It is an object of the invention to provide, inter alia, an integrable quadrature oscillator which, as compared with the known quadrature oscillator, provides a considerably more accurate phase quadrature relation between the two phase quadrature oscillator signals and which relation can the maintained over a frequency range extending from a lower limit frequency to an upper limit frequency which is many times lower and may even be negative or which is many times higher, respectively, than that of the frequency range of the known quadrature oscillator.
According to the invention a controllable quadrature oscillator of the type described in the opening paragraph is therefore characterized in that at least the said quadrature section comprises a cascade circuit of two stages arranged in a signal path between an input terminal and an output terminal, one stage comprising a first amplifier having a low-pass characteristic and the other stage comprising a second amplifier having a low-pass characteristic, said second amplifier having a feedback path and the gain of at least one of the two amplifiers being controllable for controlling the frequency of the pair of phase quadrature oscillator signals.
The invention is based on the recognition that a feedback amplifier varying in gain and having a given, fixed low-pass characteristic effectively exhibits a time constant varying with the gain and that together with a further fixed time constant, and by means of a correct gain control it is particularly suitable for shifting the desired frequency within the variation range of this effective time constant accurately through 90.degree. in a quadrature section of a quadrature oscillator.
When using the measure according to the invention, the controllable effective time constant of the second amplifier realizes a controllable phase shift in addition to a phase shift which is realized by the fixed time constant of the first amplifier so that a total phase shift of 90.degree. is achieved for a desired frequency. Whereas the gain of the second amplifier is mainly adjusted for obtaining an accurate 90.degree. phase shift for the desired frequency, the gain of the first amplifier is mainly adjusted for adjusting the loop gain at a value of one for this frequency. As a result, the frequency control range and the accuracy of the phase quadrature relation between the two phase quadrature oscillator signals is fundamentally larger than in the case of the known quadrature oscillator.
For the said gain adjustment it is not necessary for each one of the two amplifiers of the quadrature sections to be controllable, but it may be sufficient for only one of the two amplifiers to be controllable and for the other amplifier to be an amplifier/limiter (clipper) if, for example, square wave-shaped oscillator signals are acceptable or desirable SInce an amplifier/limiter operating in its limiting mode has a gain which is dependent on the input signal amplitude, the gain of the amplifier/limiter can be simultaneously adjusted by means of a singe adjustment of the gain of the controllable amplifier at such a value that as a whole a gain of one is achieved for the desired frequency in the relevant quadrature section.
The controllable quadrature oscillator according to the invention is particularly suitable in an integrated form because parasitic capacitances which are inherent in integrated circuits and which of themselves realize a low-pass filter effect can then be utilized.
The accuracy of the said phase quadrature relation is enhanced in a preferred embodiment of the controllable quadrature oscillator according to the invention which is to this end characterized in that the other quadrature section also comprises a cascade circuit of two stages which correspond to the first-mentioned two stages, each quadrature section having a signal path between an input and an output incorporating the signal path of the first amplifier and the feedback path of the second amplifier, the signal path of the second amplifier being located outside the said signal path between the input and the output of each quadrature section.
For a further increase of the frequency control range such a controllable quadrature oscillator is characterized in that at least the second amplifier of each of the two quadrature sections has a first order low-pass characteristic.
A quadrature oscillator according to the invention, which is particularly suitable for generating phase quadrature oscillator signals having comparatively high frequencies, is preferably characterized in that the second amplifier of each of the two quadrature sections is negatively fed back via the feedback path and in that for increasing the oscillator frequency the gain of both the first and the second amplifier in each of the two quadrature sections increases, and conversely.
A quadrature oscillator according to the invention, which is particularly suitable for generating phase quadrature oscillator signals having lower frequencies as compared with the previous embodiment, is preferably characterized in that the second amplifier of each of the two quadrature sections is positively fed back via the feedback path and in that for increasing the oscillator frequency the ratio between the gain of the first amplifier and that of the second amplifier in each of the two quadrature sections increases, and conversely.
In order to avoid the use of large time constants in the last-mentioned controllable quadrature oscillator according to the invention, it is preferably characterized in that the stage comprising the second amplifier has an input in common with a third amplifier having a low-pass characteristic and a fixed gain, and an output which is coupled to a first input of a subtractor stage, an output of the third amplifier being coupled to a second input of the subtractor stage for subtracting the output signal at the third amplifier from that of the said stage comprising the second amplifier, the 3 dB limit frequencies of the first and third amplifiers being higher than that of the second amplifier.
Another alternative for avoiding large time constants is characterized by a fourth amplifier having a low-pass characteristic and a fixed gain, which amplifier has a positive feedback path whose signal path is located outside, and its positive feedback path is located in the signal path between the input and the output of each quadrature section, the 3 dB limit frequencies of the first and fourth amplifiers being higher than that of the second amplifier.
For obtaining sinusoidal oscillator signals having a high spectral purity and a frequency-independent amplitude, another preferred embodiment has an automatic gain control and is characterized in that the gains of the first amplifiers as well as those of the second amplifiers are mutually equal and are controllable in mutually equal directions by means of a first and a second control signal at a common first and a second control terminal, respectively, and in that a control signal generating circuit is arranged between the pair of oscillator outputs and the said first and second control terminals and comprises an amplitude detection device for detecting the amplitude of the vectorial sum of the phase quadrature oscillator signals, said amplitude being negatively fed back to the first and second control terminals for the purpose of automatic amplitude control of the oscillator signals.
A frequency-independent adjustment of the amplitude of the oscillator signals at a desired value in the last-mentioned controllable quadrature oscillator with a small harmonic distortion is possible in a further preferred embodiment which is characterized in that the amplitude detection device is coupled to the two control terminals via an amplitude level control device, an amplitude control signal being supplied from an amplitude control input to said amplitude level control device, which signal is subtracted in the amplitude level control device from the said amplitude to be negatively fed back of the vectorial sum of the phase quature oscillator signals.
A simple realization of this preferred embodiment is characterized in that the amplitude detection device comprises first and second squaring circuits which are coupled to the respective oscillator outputs and to two inputs of an adder stage, which adder stage is coupled to the two control terminals via a subtractor stage incorporated in the level control device, the amplitude control signal being subtracted in said subtractor stage from the said amplitude to be negatively fed back of the vectorial sum of the phase quadrature oscillator signals.
A considerable simplification of the frequency control of the last-mentioned controllable quadrature oscillator with a small harmonic distortion is achieved in yet another preferred embodiment which is characterized by a frequency control device which is arranged between the amplitude detection device and the first and second control terminals to which a frequency control signal is applied from a frequency control input, said frequency control device comprising a modulating device for mutually varying the first and second control signals in dependence upon the frequency control signal.
If square wave-shaped oscillator signals are desired, it is possible, for example, to incorporate the second amplifier in each of the two sections in an amplifier/limiter circuit and its control can then be dispensed with. Since a frequency control in each of the two sections is possible by means of only the first control signal at the first controllable amplifier, the control signal generating circuit can also be dispensed with.
A further preferred embodiment of a controllable quadrature oscillator according to the invention, which supplies phase quadrature oscillator signals whose mutual phase relation reverses sign when the first control signal reverses sign, is characterized in that the amplitude detection device is coupled to the two control terminals via an amplitude level control device to which an amplitude control signal is applied from an amplitude control input, which signal is subtracted in the amplitude level control device from the said amplitude to be negatively fed back of the vectorial sum of the phase quadrature oscillator signals.
In the case of a controllable quadrature oscillator with a small harmonic distortion the modulating device should also comprise a multiplier circuit in which the afore-mentioned frequency control signal is multiplied with the amplitude to be negatively fed back of the vectorial sum of the phase quadrature oscillator signals in such a way that a reversal of the sign of the frequency signal results in a reversal of the sign of at least the first control signal and hence in a reversal of the sign of the output signal of the first amplifier in each of the two sections.
According to the invention a practical preferred embodiment of a controllable quadrature oscillator with a small harmonic distortion is characterized in that the first and the second amplifier in each of the two quadrature sections is an emitter-coupled transistor pair and a transistor pair fed back from collector outputs to base inputs, respectively, whose emitters are mutually coupled via two mutually equal emitter resistors, each transistor pair being coupled to a controllable current source arranged in a common emitter lead and the collectors of the two transistor pairs of the first and the second quadrature section being coupled to a balanced first and a balanced second oscillator output, respectively, and to the bases of the transistor pair of the first amplifier in the second and the first quadrature section, respectively.
To provide the possibility of continuous frequency control through the zero value, the last-mentioned quadrature oscillator is characterized in that each collector of the transistor pair of the first amplifier in each of the two quadrature sections is connected to a supply voltage via a parallel arrangement of a collector resistor and a voltage divider, which voltage divider comprises two series resistors having a common node to which one of the two collectors of the transistor pair of the second amplifier is coupled via a resistor.
The invention will now be described in greater detail with reference to the accompanying drawings which only serve to illustrate the invention and in which mutually corresponding elements have the same reference designations .